This invention relates to combined cycle power plants and, specifically, to a bypass arrangement that allows increased output at peak periods, but also retains optimum performance during non-peak periods.
Until recently, bottoming cycle design for combined cycle power plants was relatively straightforward. The gas turbine(s) generated a predictable amount of waste heat. Heat Recovery Steam Generators (HRSGS) converted a percentage of that waste heat to steam for re-admission into an appropriately sized steam turbine. During the last few years, however, shortages of generating capacity have caused spot prices to rise by several dollars per KWhr. As a result, owners and developers have turned to the plant designers to develop ways to generate more power during the peak periods. The result has been plant designs that include duct burners in the HRSGs which can be xe2x80x9cfiredxe2x80x9d during peak periods to generate supplemental power. These designs have solved the problem of generating substantial peak power in a manner that produces lower capital cost requirements and better heat rates than can be achieved by adding simple cycle gas turbines to the grid.
Inclusion of duct burner firing to accomplish higher output xe2x80x9cpeakingxe2x80x9d operation of the combined cycle plant, however, introduces a xe2x80x9chiddenxe2x80x9d higher cost. That cost results from operating the steam turbine at lower than rated inlet pressure, and thus a xe2x80x9cnon-optimalxe2x80x9d lower pressure during xe2x80x9cnon-peakingxe2x80x9d (unfired) operation. This pressure can be several hundred pounds per square inch (psi) lower than the rated (optimal) level for highest output during the non-fired periods of the year. These penalties can be substantial if the steam turbine is only operating in fired (peak) mode for a fraction of the year, while operating in an unfired, pure combined cycle mode (i.e., without the supplemental duct burner firing) for the remaining majority of the year. Heat rate penalties in the 25-50 BTU range can result in lower output, equating to several million dollars of Net Present Value lost due to the reduced nominal power output.
In order to hold the inlet pressure relatively constant during large swings in HRSG output, a bypass arrangement is incorporated that includes steam piping that runs from the HRSG to alternate steam turbine admission point(s) that are one or more stages downstream from the nominal high pressure inlet nozzle.
During base load operation (unfired HRSG in a non-peak period), the steam turbine operates at its rated inlet pressure at optimum output, with the bypass system closed. As duct burner firing in a peak period commences, the supplemental firing and bypass systems are opened and steam is admitted to the steam turbine through the admission connection. The extra steam flow produces more torque and hence output. Since the flow is admitted downstream of the steam turbine high pressure inlet at a lower pressure, the inlet continues to operate at its rated pressure. When the unit is switched back to base load operation, the bypass system is again closed and the cycle repeats itself with the steam turbine operating at rated pressure (optimum) while base loaded. Thus, depending upon the construction of the steam turbine, a higher peak output may be possible through the bypass arrangement, than through the more typical xe2x80x9csliding pressurexe2x80x9d inlet configuration as explained further herein.
Accordingly, in its broader aspects, the invention relates to a combined cycle power plant comprising a gas turbine driving a first generator; a steam turbine driving a second generator, the steam turbine having a low pressure section, an intermediate pressure section and a high pressure section; a heat recovery steam generator arranged to receive exhaust gas from the gas turbine and condensate from the steam turbine, and to utilize heat from the gas turbine exhaust to convert condensate from the steam turbine back to steam, the heat recovery steam generator having means for supplying steam to the high pressure, intermediate pressure and low pressure sections of the steam turbine; the heat recovery steam generator further comprising a duct burner for supplying supplemental steam to the steam turbine; wherein the supplemental steam is supplied to the steam turbine downstream of the high pressure inlet to the high pressure section.
In another aspect, the invention relates to the combined cycle plant of claim 3 wherein said supplemental steam is mixed with hot reheat steam from a drum in said heat recovery steam generator prior to admission into said intermediate pressure section.